Colours in Nature

Warning colouration

Animals that are dangerous, or foul to eat, usually advertise the fact. This is called warning colouration or aposematism. It is the exact opposite of camouflage. Warning colours tend to be some combination of red, yellow, black and white.

Alfred Russel Wallace, a British naturalist, explained it this way, in 1889:

"The animals in question are possessors of some deadly weapons, as stings or poison fangs, or they are uneatable, and are thus so disagreeable to the usual enemies of their kind that they are never attacked when their peculiar powers or properties are known.
It is, therefore, important that they should not be mistaken for defenceless or eatable species... since they might suffer injury, or even death, before their enemies discovered the danger or uselessness of their attack. They require some signal or danger flag which shall serve as a warning to would-be enemies..." 

Wallace predicted that birds and other predators would reject conspicuous prey whilst accepting cryptic prey. Later reports confirmed this.

Animals with warning colours move slowly and expose themselves to sight. The sluggishness and exposure helps to advertise their defence. Along with the colour and behaviour often goes the foul smell of their chemical weapons. Noxious caterpillars often have thick, leathery cuticles which help them to resist young birds making a 'test'. When the bird takes a peck, disgusting fluid seeps out from special glands on its back. The caterpillar (or other larva) will often survive such an attack, and the young bird has learnt a lesson it will never forget. All in all, the predator is given good warning. Tests show that warning colours definitely do deter predators.

Some individual animals will die or receive damage while birds or mammals on the attack learn about the connection between colour and taste. However, if warning costs less than hiding, the animal benefits. And the advertising traits such as colours may serve other functions as well. The patterns may help mate identification within the species, for instance.

Warning colouration is the basis of two different kinds of mimicry: Müllerian mimicry and Batesian mimicry.

Bright colours

Animals can, and do, show bright colours which have other functions. The most common functions are:

• Identification, a secondary sex characteristic to find a mate.
• Temperature regulation and protection
• Camouflage against unusual backgrounds.

However, the bright colours used for these functions tend to be different from the standard warning patterns and colours.

Use of pigments for camouflage or defense

Pigmentation is used by many animals for protection, by means of camouflage, mimicry, or warning coloration. Pigments such as melanins in the skin may serve to protect tissues from ultraviolet radiation.

The skin of an octopus changes almost immediately as it moves over different colored areas of the ocean floor. Color changes are controlled by small, elastic, pigment-filled sacs, known as chromatophores, found in the outer layer of the skin. The sacs are filled with red, yellow, or brown pigment, with some species having as many as five colors. When the muscles attached to each chromatophore contract, they pull the pigment sac outward to reveal an area of a particular color. When the muscles relax, the chromatophores close back up. The ink clouds a cephalopod produces complement this line of defense.

A secondary sex characteristic to find a mate
Pigments may also aid in sexual reproduction, by identifying species and gender of animals to potential mates, or signaling readiness to breed. It is now believed that chameleons change color not so much to blend into the surroundings, but rather to signal, fend off rivals, or attract a mate.

Temperature regulation and protection by colour

Some animals are coloured for physical protection, such as having pigments in the skin to protect against sunburn, while some frogs can lighten or darken their skin for temperature regulation.

Some frogs such as Bokermannohyla alvarengai, which basks in sunlight, lighten their skin colour when hot (and darkens when cold), making their skin reflect more heat and so avoid overheating.

Unknown function of colours

The colouring of many species has unknown function. This is usually because their life habits have not been studied sufficiently, and because no field tests have been done. One example is the extraordinary moth Utetheisa pulchella, the Crimson Speckled Moth. Its association with the plant Dittrichia viscosa, which has a somewhat unpleasant smell, suggest the moth has warning colouration, but the matter is not settled. The related species Utetheisa ornatrix, which feeds on other plants, is known to exude (squeeze out) alkaloids to put predators off.